1. Field of the Invention
This invention pertains generally to the field of printing and decorating processes and more particularly to the use and mounting of doctor blades in connection with printing processes.
2. Description of the Prior Art
Doctor blades are used in a variety of printing processes, most commonly in the rotogravure or intaglio type processes. The printing design is formed in recesses on a printing plate with ink being applied to the printing plate to fill the recesses. The excess ink must usually be removed with a blade of some sort so that ink will be applied to the printed surface only from the recessed areas of the plate. Depending on the printing technique, the doctor blade may be employed as a wiper, in which the blade is oriented away from the direction of rotation of the roller, or as a scraper, in which the blade meets the roller in an orientation which is against the direction of rotation of the roller. As used herein, the term doctor blade will refer to either a wiper or scraper type blade.
Doctor blades may also be utilized in other printing processes. For example, in flexographic printing where the amount of ink in each cell of an anilox metering roller must be precisely controlled, a doctor blade may be utilized to remove excess ink from the anilox metering roller.
In the utilization of a doctor blade for either scraping or wiping the surface of the roller, it is desirable to maintain a reasonably constant pressure of the blade against the roller as the roller rotates, as well as providing uniform pressure along the entire length of the blade. The uniform pressure of the blade against the roller provides an even coating of the surface of the roller with ink, so that the work being printed does not have portions thereof with unusually heavy coats of ink while other portions of the printed work do not have enough ink. The pressure of the doctor blade against the roller must be uniformly maintained despite the displacements and compressions of rollers which take place when the various rollers of a press are engaged to begin printing. Often, the typical fixedly mounted doctor blades will not follow the distortions of the roller and will tend to apply greater pressure to one part of the roller than to other parts of the roller. The result is the distorted application of ink on the printed object as described above. The problem of displacement or rollers becomes particularly evident where the rollers are cantilever mounted, since such rollers have less rigidity than rollers which are mounted at both ends.
The pressure of the doctor blade on the roller should be substantially constant despite small displacements of the roller and the incremental wearing away of the edge of the doctor blade and possibly the surface of the roller. In prior doctor blade mounting systems, this pressure has typically been controlled by providing a thin doctor blade which is bent against the surface of the roller to apply spring pressure. Other methods of maintaining the pressure of the blade against the roller have been utilized. For example, the blade may be mounted so that it can be rotated into contact with the roller, with either springs or hydraulic cylinders applying a controlled amount of torque to the mounting of the doctor blade to control the pressure of the doctor blade against the roller. With the use of these mounting techniques, the doctor blade can generally move in only one degree of freedom, rotation about a longitudinal axis, i.e. an axis parallel to the long direction of the doctor blade and the axis of rotation of the roller. Such systems have inherent limitations in that they allow adjustment only of the pressure of the entire doctor blade against the roller, and do not allow adjustment of the pressure of portions of the doctor blade to account for misalignments between the axes of the doctor blade and the roller, and minor displacements of the roller. Such systems also have difficulty maintaining relatively light pressures of the doctor blade on the roller surface.